Optoelectronic devices such as transmitters and receivers commonly comprise an optoelectronic chip within a housing (the “package”). The chip is optically coupled to an input and/or an output optical fibre that passes through a wall of the housing. Between the optical fibre and chip there may be intervening bulk optical components, such as lenses, optical splitters, and optical isolators. The chip may be an optoelectronic modulator, such as a Mach-Zehnder modulator chip or an electro-optic absorption modulator chip. Alternatively the chip may be another optoelectronic chip, such as a semiconductor laser chip or a photodetector chip.
The housing provides protection to the chip and other components from mechanical damage during higher level assembly into modules, transport or connection into an optical communications network. Further, the housing is typically sealed and provides environmental protection from ambient humidity, chemical degradation and light. In many cases the housing may be hermetically sealed to provide the highest level of environmental protection. U.S. Pat. No. 7,066,658 discloses an arrangement by which the optical fibre passing into a housing may be hermetically sealed directly to the inside of an opening through a wall of the housing. U.S. Pat. No. 6,712,528 discloses an arrangement in which an optical fibre entering a housing is hermetically sealed into a ferrule which also passes through the opening in the housing wall, the ferrule is hermetically sealed onto a protective snout, and the snout is in turn hermetically sealed to the outside wall of the housing, around the opening.
The housing also provides a robust body to which both the chip and other components may be connected, such as bulk optical components and a thermo-electric cooler (which may be connected between a chip and the internal floor of the housing). In particular, the housing provides a robust fixture to which the optical fibre is connected.
Frequently the applications within which such components are deployed are size sensitive, with requirements to minimize the “footprint” of the housing. This can be particularly important when the housed device is integrated into a higher level optoelectronic module, e.g. a transmitter, receiver or transponder module that is provided with electronic control circuitry for the housed device.
Attention to the reduction of the size of componentry within optoelectronic housings has to date focused on the reduction in size of bulk optical components within the housing. U.S. Pat. No. 7,161,725 and US2007/0091300 disclose reduced footprint bulk optical components. However, reduction in the size of the components within a housing can only achieve a limited reduction in the corresponding size of the footprint of the housing required to contain them. Similarly, the reduction in the size of the other components within a housing can only provide a limited amount of extra space to facilitate the use of a longer chip. In particular, such reductions in the size of bulk optical components can only achieve limited reduction in the corresponding length of the housing, or a limited increase in the length of the optoelectronic chip that can be housed within a housing matching an existing footprint. Further, the requirement for access into the housing by alignment apparatus that positions some of the components also limits the possible reduction in the width of the housing.
Two different arrangements are known for optically coupling an optical fibre to the optical waveguide of an optoelectronic chip.
In a first known arrangement, known as “butt-coupling”, the optical fibre passes in through the housing wall and the end of the fibre is connected to the chip. The fibre end may abut the chip, being directly bonded onto the chip by means of optical adhesive, such as optical resin. Alternatively, an optical coupling element may be provided bonded to the end of the optical fibre, and the optical coupling element is bonded to the chip, such that both the optical fibre and optical coupling element are butt-coupled to the chip. Such an optical coupling element may be a short homogenous cylinder of glass, and facilitates handling of the fibre end during the alignment stage of device assembly. U.S. Pat. No. 7,228,014 discloses the use of an optical coupling element in the butt-coupling of an optical fibre to a chip.
When butt-coupled to a chip, an optical fibre is only required to be aligned in two dimensions, being the orthogonal directions in the plane of the bonding surface of the chip. Disadvantageously, alignment of the optical fibre end within the housing requires a minimum housing width, such that fibre end alignment equipment can grasp and manoeuvre the fibre end within the housing. FIG. 1 illustrates a device according to this first known arrangement, showing a cross-sectional view through the device in the plane of the optoelectronic chip and optical fibre.
In a second known arrangement, known as “air-lens alignment”, an optical lens is bonded into an opening in the wall of the housing, and the chip and the optical fibre are aligned on opposite sides of the lens, such that light from one is focused onto the other by the lens. In such an arrangement the chip may be connected to the floor of the housing, following which the fibre end is aligned in three dimensions to a corresponding position on the other side of the lens. A separation is required between the chip and the lens for correct focusing of light passing between the chip and optical fibre, and in the air-lens alignment arrangement, where the lens is in the housing wall, that separation is provided within the housing, which disadvantageously increases the length of the required housing. FIG. 2 illustrates a device according to this second known arrangement, showing a cross-sectional view through the device in the plane of the optoelectronic chip and optical fibre. For clarity, the external feed-through, strain relief collar and a protective tube (shown in FIG. 1) have been omitted from FIG. 2.
There remains a need for an optoelectronic device having a reduced footprint, or which facilitates the use of a longer optoelectronic chip within a housing having an existing footprint. In particular, there remains a need for an optoelectronic device having a reduced housing length and/or housing width, or which could facilitate the use of a longer and/or wider optoelectronic chip within an existing housing.